In fabricating semiconductor devices such as transistors, ICs and LSIs, electrodes on a chip are connected to external leads by gold wires. Semiconductor devices are typically fabricated by the following steps:
(a) prepare a lead material from a strip of Cu or Cu alloy or Ni or Ni alloy having a thickness of 0.1 to 0.3 mm;
(b) stamp out a lead frame conforming to the shape of the semiconductor device to be fabricated;
(c) apply high-purity Si or Ge semiconductor elements to selected areas of the lead frame by thermocompression with an electrically conductive resin such as Ag paste or through a plating of Au, Ag, Ni or their alloy formed on one surface of the lead material;
(d) connect the semiconductor elements to the lead frame by gold wires (this is a bonding step);
(e) enclose with a protective plastic package the semiconductor elements, gold wires and parts of the lead frame to which the semiconductor elements have been bonded;
(f) remove unnecessary parts from the lead frame to form discrete leads; and
(g) apply a soldering material to the legs of the leads to enable connection of the semiconductor device to the substrate.
In the bonding step (d), the gold wire is fixed sequentially at the proper points of the semiconductor elements and the lead frame (kept at 150.degree.-300.degree. C.) by a manual or automatic bonding machine. That is, first, the gold wire is heated at its tip with an oxy-hydrogen flame or by electrical means to let it take the form of a ball, which is pressed against the semiconductor, then an extension of the gold wire is pressed against a point on the lead frame to be fixed thereon and the wire is cut to finish the bonding cycle. Then, this cut end becomes the tip serving as a point to be pressed against the next bonding point on the semiconductor in the subsequent bonding cycle.
As bonding at higher speed and more highly integrated circuits are desired, the use of finer and stronger gold wires is necessary. But the currently used wires are made of pure gold which has a relative low tensile strength at room and high temperatures and cannot be drawn to a smaller diameter of 0.05 mm or less without accompanying frequent breakage of the wire, and even if pure gold could be drawn into wires that fine, they would often break during the bonding step. What is more, because of the low softening point of pure gold, the crystal grains of the wire being cut with a flame or by electrical means recrystallize to become bigger and brittle, and when the gold ball is pressed at between 150.degree. and 300.degree. C., the bonded wire softens to deform the wire loop connecting the semiconductor elements and lead frame and may cause shorting. The pure gold wire also does not have satisfactory bond strength with respect to the semiconductor elements and lead frame.
Gold bonding wires should have the highest possible content of gold in order to make most use of its physical, electrical and chemical properties. However, wires with a gold content of 99.999% or more having a diameter of 50 .mu.m or less have a tensile strength of only 6 to 7 g at room temperature and have a still lower value at elevated temperatures.
Therefore, the primary object of the present invention is to provide a gold wire which (1) has a very high gold content and (2) is sufficiently protected against increased brittleness and deterioration in other properties, and which (3) claims tensile strength values at room temperature and elevated temperatures which are at least 50% higher than those previously attainable even when the wire is drawn to a diameter of 50 .mu.m or less.